Non-destructive testing (NDT) is a widely-used method for determining surface defects in manufactured equipment and welding applications. One NDT technique utilizes a red dye penetrant which enters into small surface flaws in order to make them more visible. This testing method, called liquid penetrant examination (LPE), is a popular technique since it easily reveals previously hard-to-see cracks, corrosion pits, or other surface damage in the manufactured parts. For enhanced contrast and detection of smaller surface flaws, fluorescent penetrants that emit a green glow when illuminated with ultraviolet (UV) light are frequently used.
LPE is particularly useful in examining machined parts on weaponry, where early identification of surface flaws may help prevent a catastrophic failure due to the high pressures and high temperatures that can exist inside the weapon. LPE is also commonly used to detect flaws in welding, where small fractures in welds can result in failure of the weld. NDT is preferred over other quality inspection techniques (which may require damaging the part), since the tested material remains useable after NDT (if no defects were detected).
Photoluminescence (PL) is the emission of light (electromagnetic radiation, photons) after the absorption of light. It is one form of luminescence (light emission) and is initiated by photoexcitation (excitation by photons). Following photon excitation, various charge relaxation processes may occur in which other photons with a lower energy, compared to the excitation source, are re-radiated on some time scale. The energy difference between the absorbed photons and the emitted photons (also known as Stokes shift) can vary widely across materials from nearly zero to 1 eV or more.